This invention relates to a method, and to the product thereof, whereby a product having a zinc-aluminum containing surface layer, preferably sheet and strip, is treated to produce a thin, uniformly black, light-absorptive surface layer on said product. The method represents an economical system for producing corrosion resistant material for solar collectors. This efficient, yet economical method for producing such material is particularly important as a result of the rising concern over the serious depletion of and access to the world's energy producing natural resources. This, however, is not to suggest that such concern is only recent.
It has long been recognized that alternative energy sources were vitally necessary to the well being of mankind. However, the problem was not so much in finding such alternative sources, but in developing a safe, efficient cost-competitive system for using the alternative sources. The method of this invention, which results in an inexpensive, corrosion-resistant, light-absorbing sheet product suitable for use as the absorbing surface of solar collectors, is a major step in the right direction.
Solar collectors by their very nature are exposed to the atmosphere. As a consequence, an inexpensive base material resistant to atmospheric conditions, while affording galvanic protection to cut or exposed edges of solar collector components, is a necessity. Zinc-containing coated ferrous strips, such as galvanized and aluminum-zinc alloy coated strip, are ideal candidates for solar collector components as they offer both galvanic protection and resistance to atmospheric conditions to the ferrous base. However, in order to effectively use such coated products for construction of a light-absorbing component of a solar collector, a means is required to confer light-absorbing properties to the component's surface. Heretofore, heavy reliance has been placed on expensive treatments or paints, often with high application costs, such as black chrome electroplate and black pigmented paints for this purpose.
However, there are other known procedures and compositions which are available for blackening metals. None were found, for instance, which employ processing steps herein described for the rapid and continuous blackening of aluminum-zinc alloy coated products. U.S. Pat. No. 3,391,012 (Mitchell et al) teaches an electroless process for blackening of aluminum. U.S. Pat. No. 3,899,367 (Mitchell) teaches compositions and methods adapted for blackening hardened steels. U.S. Pat. No. 3,314,812 (Mitchell) teaches a method of blackening stainless steels. Finally, U.S. Pat. Nos. 3,127,279 (Baig et al) and 2,679,475 (Singler) teach processes for blackening a variety of metals including zinc, zinc alloys and galvanized zinc. Singler discloses no special surface preparation and indicates that the blackening treatments require minutes rather than seconds. Such a process is hardly suitable for a continuous operation. Baig et al, on the other hand, teaches various surface treatments, all of which are acid. Even with such acid treatment of the surface, the subsequent immersion time in the blackening solution is at least 30 seconds up to several minutes. In contrast to such prior art practices, the present invention represents a method of applying an inexpensive light-absorptive surface layer to an inexpensive corrosion resistant, aluminum-zinc alloy coated base, preferably a ferrous base, in a rapid and continuous manner, to provide a product suitable for solar collector components.